FIG. 4 shows a known amplifier circuit comprising a differential input amplifier stage 4 and a push-pull output stage 8. With this amplifier circuit, an input signal from an input signal source 1 is amplified by the input amplifier stage 4 which comprises first and second transistors 2, 3 having their emitters connected to each other. The resulting output signal is applied from the collector of the second transistor 3 to a drive stage 5, which in turn produces two signals out of phase with each other by 180 degrees. These signals are applied to third and fourth transistors 6, 7 constituting the push-pull output stage 8. A pushpull signal obtained at the output point A of the stage 8 is fed to a speaker 10 via an output coupling capacitor 9 to drive the speaker 10.
With the above circuit, the signal from the input signal source 1 is fed to the base of the first transistor 2 via an input coupling capacitor 11. The base has connected thereto a bias circuit 16 comprising first to third resistors 12, 13, 14 and a decoupling capacitor 15. Connected between the output point A of the output stage 8 and the ground is a negative feedback circuit 20 comprising fourth and fifth resistors 17, 18 and a negative feedback capacitor 19. The point of connection, B, between the fourth resistor 17 and the fifth resistor 18 is connected to the base of the second transistor 3.
Accordingly, the a.c. input signal from the input signal source 1 and d.c. bias voltage from the bias circuit 16 are applied to the base of the first transistor 2, while the a.c. signal from the output point A, as voltage-divided by the fourth and fifth resistors 17, 18, is applied to the base of the second transistor 3, with d.c. voltage at the output point A also applied as it is to the same base.
When the first resistor 12 and the second resistor 13 are made equal in resistance value, one-half of the power supply voltage Vcc is available at the point C shown. Further if the third resistor 14 is made equal to the fourth resistor 17 in resistance value, the d.c. voltage at the output point A also becomes equal to one-half the power supply voltage Vcc through the negative feedback action. Consequently it is possible to maximize the range in which the output signal at the output point A is free of distortion, i.e. the dynamic range of the output signal.
However, if it is attempted to fabricate the circuit of FIG. 4 in the form of an integrated circuit, the input coupling capacitor 11, output coupling capacitor 9, decoupling capacitor 15, negative feedback capacitor 19, etc. must be arranged externally of the integrated circuit. This gives rise to the problem of increasing the number of external parts. Furthermore, the connection of these external parts to the integrated circuit entails the problem that many external pins need to be connected to the circuit.
Further if one end of the fifth resistor 18 is merely grounded to eliminate the feedback capacitor 19 and thereby reduce the number of external parts and external pins, it becomes no longer possible to effect the negative feedback operation normally and to maintain the voltage at the output point A at Vcc/2 because the impedance of the base of the second transistor 3 lowers. Alternatively if it is attempted to give an increased value to the fifth resistor 18 with the negative feedback capacitor 19 removed so as to maintain the output point voltage at Vcc/2, the amount of a.c. feedback increases to greatly decrease the voltage gain of the amplifier circuit, while it is then difficult to incorporate the fifth resistor 18 into the integrated circuit.